chrillek:
I’m not computer-savvy enough to understand what is in the system logs. I do think I’ve solved the problem, though. I’ll know if the drive behaves nicely for 24 hours.
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Thanks to everyone who offered their suggestions to help me resolve my problem. I’ve reached the conclusion that the issue boils down to Apple’s less-than-optimum support for multiple external drives plugged into USB3 ports. I’ve ordered a Thunderbay 4 RAID enclosure and 4 4TB Toshiba HDDs from OWC and when I get that set up, I expect all of my bizarre issues will disappear.
I also bought a copy of OWC’s SoftRAID, which is much better than DiskUtility in setting up RAIDs. It also has the ability to monitor the health of the drives in the RAID so that individual drives that are beginning to fail can be replaced before they do fail.
This has certainly has been a steep learning curve but I’ve learned a lot!
We’re hopeful this resolves your issue and finally puts this all to rest.
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If I understand all that correctly, you’re now buying 4 disks of OWC after you have concluded that there’s a hardware problem somewhere with the OWC disk you already have?
It’s your money, of course.
If it were my hardware, I’d make sure that the problem is neither with the cable nor the disk nor the Mac‘s USB port(s) before I buy another piece of (expensive) hardware from the same company.
4 disks with 4TB each – that gives you 12TB with RAID 5…
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chrillek,
Well, no, I never thought I had a hardware problem, despite what members of this forum thought. I didn’t know what kind of problem I had but I think I have figured it out now. I reached out to the support at Carbon Copy Cloner and the answer I got there pointed to my conclusion. Neither of my main external drives has ever exhibited the bizarre symptoms I’ve encountered and all of them have passed, multiple times, the Disk Utility First Aid routine. Yes, it is my money but I’m spending less than what a new Mac costs and, if nothing else, I will have a lot more free desktop space! 
I do think it would be useful for the support team to consider the possibility that those of us who have their databases on an external drive might give some thought to setting up a RAID. RAID 1 requires a minimum of 2 drives. The second drive is a mirror copy of the first. A 4 bay RAID enclosure gives the option of setting up two RAIDs in the same enclosure. I’ll look at RAID 5 again, but I think RAID 1 will be sufficient for my needs. As always, thank you for your thoughts.
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RAID is certainly a good idea for any important data, but I am not sure that will solve your current problem.
To the extent Apple support for multiple external drives is “less than optimum” that is related to throughput speed which is shared by all of the drives. This can worsen rather than improve with a RAID array.
What type of Mac are you running? Perhaps an upgrade of the computer running this is in order so that you can handle your growing storage needs?
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rkaplan:
I’m running a Mac Mini with an M1 chip and 16Gb of RAM and the OS is Sequoia 15.6.1. I will find out later this week if a RAID solves my problem or not. If nothing else, I will have more space on my computer desk! 
That should be able to achieve your mission.
I agree with @chrillek - this is likely a hardware problem, possibly as simple as a cable that needs to be replaced.
Setting up a RAID array may well be a good idea but it is unlikely to address the issue prompting your original post.
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Setting up a RAID is not a good idea in your case. On top of the issues you are encountering, you will add the complexity of having yet another software and disk management layer.
I have an OWC Thunderbay, had it for years, it’s solid. I just use it as a JBOD - independent drives, some SATA, some SSD. One of the SATA drives is partitioned (2 data partitions).
For backups to it (and other local drives) I use CCC. And I have scheduled backups that are separated by at least 1 hour in the wee hours.
For mission critical data, I never use partitioned (i.e., more than one Finder data volume) per disk. Why? because I try to keep things as simple as possible.
If I were doing video, etc, I’d be using RAID, but I’m not.
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